Pulse signaling system



NOV. 12, 1946. J, EVANS 2,411,130

PULSE SIGNALING SYSTEM Filed Aug. l2, 1943 3 Sheets-Sheet 1 Jox/N E Wwm A T 70E/VE Y Nov. 12, 1946. J. r-:vANs

PULSE SIGNALING SYSTEM Filed Aug. l2, 1943 5 Sheets-Sheet 2 me m@ \S\ i -Mwah J. EVANS 2,411,130 PULSE S'IGNALING SYSTEM Filed Aug. l2, 1943 3 Sheets-Sheet 3 l /q fc.//p/?0x.

Nov. 12, 1946.

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Patented Nov. 12, -1946 PULSE SIGNALING SYSTEM John Evans, Kingston, N. J., assignor to Radio Corporation of America, a corporation of Dela- Ware Application August 12, 1943, Serial No. 498,283

11 Claims. 1

The present invention relates to a method of and apparatus for modulating a device inherently requiring a constant amplitude of voltage. In a more specific aspect, the invention relates to a pulse transmissionsystem, and particularly to a method of and apparatus for generating pulses of ultra high frequency energy by means of a magnetron, and for varying the time of occur- `rence of these generated pulses in accordance with modulating potentials.

A requisite condition in employing a multiple resonant cavity type of magnetron to efficiently produce pulses of ultra high frequency energy is 1 that the voltage to be applied momentarily to the magnetron for keying 0r firing the same should have a constant amplitude. This voltage should of short duration havessentially flat. The pulse system of the `present invention provides means for satisfying this condition and also means for varying the number of generated pulses of ultra high frequency per modulation cycle without changing the average pulse rate per second. Putting it in other words, the present invention describes a system for reum shape of pulse to be supetron and for causing the pulse grouping to be a function of the instantaneous amplitude of the signalling component.

In accordance with the embodiment of the present invention to be described herein, a sinusoid of voltage of varying frequency (as would be delivered for example by the output of a voice operated microphone or the voltage derived from a picture scanning system) is caused to modulate the output of a saw-tooth generator circuit, so that the starting time of the saw-tooth Wave is variable. The output of the saw-tooth generator is delivered to a pulse shaping circuit Whose purpose is to shape the pulses to a shape optimum for the operation of the magnetron. The variable time spaced pulses are caused to key a magnetron, as a result of which there are obtained pulses of radio frequency energy of constant amplitude and shape but which vary in time spacing. The pulses generated by the magnetron are transmitted over a suitable medium, such as by an antenna or wave guide, for example, and these transmitted pulses may be received plied to the magn on any suitable pulse receiving equipmentsuch f' as one wherein the received pulses are caused to operate a phase sensitive circuit precedingthe Ai more detailed description of the invention follows in conjunction with a drawing, wherein:

Fig. 1 represents, in schematic outline, one embodiment of the invention applied to a transmission system;

Fig. 2 illustrates a circuit diagram following the outline of Fig. 1; I

Fig. 3 graphically shows, by means of two curves a and b, the operation of the saw-tooth generator of Fig.2;

Fig. 4 graphically illustrates, by means of two curves c and d, the output of the modulator of Fig.. 2 and the nal effect of the trigger tube and its associated circuits; and i Fig. 5 illustrates, by way of example, one form of receiving circuit for receiving the pulses transmitted by the system of Fig. 2.

Referring to Fig. 1- in -more detail, there is shown a pulse transmission system in accordance with the inventionwhich comprises a suitable oscillator V1 supplying sine Wave energy, of let us say 50 kilocycles, having a wave form I0 to a differentiating amplier V2. This differentiating amplifier is biased to cut-olf in the absence of input signaling currents. The input signal of wave form I0 is of sufficient amplitude 'to operate amplier V2 at saturation. The output circuit of the diierentiating ampliiier is tuned with the stray capacity of the circuit elements and the interelectrode capacitance of the tube to a frequency of live megacycles. As a result of this, the positive peak of the input wave of form I0 triggers off the five megacycle resonant circuit in the output of the differentiating amplifier V2 to produce a damped five megacycle train of wave form I I. It .should be noted that this wave form essentially comprises a substantially sinusoidal ve megacycle Wave of large amplitude followed by several smaller waves of diminishingl amplitude. The Wave form II is impressed upon the diierentiator IDD comprisinga suitable seriescapacitor-shunt resistor arrangement to produce a wave :form I2 having a sharp positive peak followed by a sharp negative peak. Wave form I2 is impressed upon a saw-tooth generator V3.

` This saw-tooth generator is so arranged that the positive peak of the diiferentiator pulse I2 causes the generator V3 to discharge its condenser to produce a wave form such as I3. Wave form I3 is impressed upon a saw-tooth modulator V4 together with a sinusoid of wave form I4 from a suitable modulation source I IIJ. Modulation source IIS) may be a voice operated microphone or a picture scanning system for supplying a sinusoid of voltage of varying` frequency. The modulator V4 is biased to cut-off for the maxi- .mum saw-tooth voltage from the generatorfVs 'of the grid voltage Eg.

3 in the absence of the impressed sinusoid of voltage I4. It will thus be seen that the modulator tube V4 passes current substantially solely on the positive peaks of the sine wave I4, on which are superimposed the saw-tooth oscillations I3. This is shown graphically at I5 and I5. The output of the saw-tooth modulator V11, as graphically represented at I5, illustrates that with varying values of total grid voltage Eg (on the saw-tooth modulator) corresponding with varying amplitude of the sinusoid, there will be correspondingly' differently positioned pulses of current Ip starting at different points of the Eg--Ip characteristic. Graph I5' illustrates the superposition of the saw-tooth oscillations upon the sinusoid. The sinusoid plus the saw-tooth components of graph I5 determine the peak instantaneous value The output of saw-tooth modulator V4, which is impressed on the current limiter V5, comprises a series of pulses whose groupings are a function of the instantaneous amplitude of the sinusoidal voltage I4. The current limiter V5 serves to remove small amplitude variations so as to provide pulses of uniform amplitude whose leading edges or time ofvoccurrence, so to speak, are functions of the sine wave modulation amplitude. This is graphically represented at I6, whereinthe vertical lines represent different positions which the pulses may take, depending upon the instantaneous amplitude of the sinusoidal voltage represented by the marks on the ordinate Eg. current limiter V5 is impressed upon a trigger tube Vs to which is connected a pulse shaping network |20. This network is preferably an artificial line'comprising a plurality of sections of inductance and capacitance. The constants of the pulse shifting network |20 determine the shape of the output pulse of the trigger tube V6. The trigger or discharge tube Ve is caused to pass current by the application of the leading edge of the pulses applied to it from the output of the limiter, at which time the pulse shaping `circuit will give up its energy at a finite rate to the magnetron |30. The voltage pulses applied to the magnetron |3 by the trigger tube are shaped to have a rapid rise, a flat top and a rapid decay, such as shown by the wave form I'I. This voltage pulse I'I, it will be seen, has extremely steep leading and trailing edges and serves to supply to the electrode of the magnetron |30 momentarily, for the duration of the pulse, a polarizing potential of constant amplitude which is sufficiently large to cause the magnetron to fire or oscillate for the duration of the pulse. The output of the magnetron |35 is in the form of a series of pulses I8 of radio frequency energy spaced in accordance with the spacings of the pulses I1. The output of the magnetron may be transmitted by a suitable line, such as a wave guide, to an antenna |40 for radiation to a suitable remotely located receiver.

Fig. 2 shows the circuit diagram for the system of Fig. 1. Vacuum tube V1 with its associated circuits represents the 50 kilocycle oscillator. The sine wave output represented by Ill is taken from the anode of vacuum tube V1 and supplied to the diierentiating amplifier V2, in whose output is a circuit tuned to iive megacycles. Tube V2 is biased to cut-onc in the absence of an input wave from the oscillator V1, and requires an input signal of sufficient amplitude to operate the tube V2 at plate current saturation. The output of the differentiating amplier V2 is shown by Wave form I I which is applied to the differentiator comprising a condenser |56 and a resistor I5|. This diiferentiator converts the first sine wave of large amplitude of wave form II to positive and negative peaks represented by wave form I2, the latter in turn being impressed upon the grid of tube V3. Tube V3 forms part of a saw-tooth generator having in circuit therewith an RC network composed of condenser C1 and resistor R1 whose time constant is approximately 2 1O5 second. The grid bias of tube V3 is adjusted for class B operation. It will thus be seen that the condenser C1 is charged through the resistor R1 from `the positive terminal of a source of anode polarizing potential, as shown, and that this charge on C1 will remain thereon until the tube V3 passes current, at which time the condenser C1 will discharge through tube V3. The positive peak of the pulse I2 applied to the grid of tube Vs by the differentiator is of suflicient value to overcome the grid bias of the sawtooth generator and to cause the condenser C1 to discharge through the `tube V3 to produce a saw-tooth pulse I3, which is supplied to the grid of the modulator V4. The saw-tooth generator thus produces saw-tooth waves at .2 microsecond intervals. The saw-tooth wave I3 represents a voltage wave produced across the resistor R1.

The positive difierentiator pulse I2 is supplied to the grid of the saw-tooth generator to cause the discharge of the condenser C1 at the completion of each charge cycle.

The operation of the saw-tooth generator may be best understood by reference to the graphs of Fig. 3, wherein curve a represents the saw-tooth wave form showing the linear rate of charge of the condenser C1. The/charge on condenser C1 reaches its maximum value once for each charge cycle, at which time the positive impulse from the diiferentiator circuit represented by wave form b impressed on the grid of the tube V3 causes the condenser to discharge. The graph a represents the voltage wave pulses across the resistor R1.

The modulator V4 is a tube operating in the class B condition. This modulator is 'biased to cut-off for the maximum saw-tooth voltage I3 and thus requires a positive vvoltage greater than a saw-tooth voltage to cause it to pass current. The modulator also has impressed on its grid a sinusoidal output represented by wave form I4 from a speech amplier V7. A suitable lsource of speech waves representing the output of a voice operated microphone or the voltage derived from a picture scanning system is impressed on the grid of the amplier V7. The modulator V4 will pass current substantially only on the positive peaks of the sine wave I4, on which will be impressed the saw-tooth oscillations I3. Thus, it will be seen that each saw-tooth pulse from the generator V3 produces a corresponding pulse in the output of the modulator V4. Since the pulses of the saw-tooth wave form I3 are superimposed on the modulatingrsinusoidal wave I4, there will be a multiplicity of pulses in the output of the modulator V4 for each positive half cycle of modulation from amplifier V1.

The graph I5 shows the plate current Ip of the modulator V4 as a function of the modulation Voltage Eg on the grid of a modulator. The modulation voltages are represented by small horizontal marks on the ordinate Eg, while the plate currents are represented by the points of I4 is essentially as shown in Fig. 4, graph c. An inspection of Fig. 4, graph c, will show that as the modulation voltage increases over the positive half of the modulation cycle of the sinuoid M, the spacing between pulses in the output of the modulator increases. During the rising and falling period of this positive half cycle of operation, the pulses are grouped closer together than during the intermediate portion of the positive half cycle of oscillation, and the spacing between pulses varies as a sine function. These pulses are represented by the vertical dotted lines of graph c. It will be evident that the pulse grouping in the output of the modulator will therefore be a function of the instantaneous arnplitude of the modulation signaling component from the speech amplifier V7. The graphs of this Fig. 4 will be discussed later in this description.

The output of the modulator V4 comprising the variably grouped pulses is impressed on the limiter V5 which serves to remove small amplitude variations to provide, in its output, pulses of uniform amplitude whose leading edges or times of occurrence are functions of the sine wave modulation amplitude. The current limiter V5 is so biased as to pass current at all times. The graph I6 represents by means of the vertical lines the dierent starting times of successive pulses impressed on its grid, depending uponthe amplitude of the sine wave input represented by the grid voltage Eg.

The anode circuit of the limiter V5, containing the uniform amplitude and variably grouped pulses, is connected to the grid of the trigger or discharge tube V6. Trigger tube V6 is normally biased to cut-oii' and has associated with its anode'a pulse shaping circuit in the form of an artificial line composed of a series of sections of small value inductors L, L shunted by condensers C, C. The constants of the condensers and the inductors determine the shape of the output pulses from the trigger Vs. The artificial line is charged to a suitable value by a positive potential supplied to one end through a charge H reactor |60. Although this charging reactor has been shown as a coil, it can be replaced by a resistor, although this is not preferred since a resistor would be ineiiicient on account of the losses produced thereby. The trigger or discharge tube V6 is caused to pass current by the application of the leading edge of a pulse applied to its grid from the limiter V5. at which time the articial line pulse shaping circuit will give up its energy at a finite rate until the energy stored on condensers C, C in the different sections of the line is dissipated. The moment the trigger V6 passes current, the artificial line applies a pulse of polarizing potential of steep rectangular wave form to the magnetron |30, thus causing the magnetron to pass current for the duration of the pulse applied thereto by the trigger. i

The magnetron comprises a known type of ultra high frequency electron'discharge device generator utilizing a resonant cavity in its anode. Such a magnetron may, for example, be of the type described in I-Iansell Patent 2,217,745, granted October l5, 1940. The magnetron is supplied with an electromagnetic field shown by the circular dotted line for supplying iiux in a direction parallel to the cathode. The cathode is shown connected to ground while the anode is shown connected by means of a lead to the cathode of the trigger tube V6. Output from the anode is supplied by means of a connection |35 connected at one end by means of a loop to the interior of the resonant cavity of the magnetron and connected at its other end to a suitable radiator such as a dipole antenna or an electromagnetic horn. Connection |35 may be a wave guide 5 or a concentric transmission line. This type of magnetron requires a rectangular wave pulse of extremely steep slopes for optimum o peration, and 'this Wave form should have a iinite operating voltage determined by the magnetic field of the magnetron. The trigger V6 sup-plies the required finite voltage for operating the magnetron in pulses.

In the operation of the system of the invention,

it will be seen that the number of pulses applied to the trigger tube Vc per cycle of operation varies in accordance with the ratio of modulation frequency to the oscillator frequency of Vi. Thus, if the oscillator frequency from V1 is 50 kilocycles and the modulation frequency from tube V7 is 1000 cycles per second, then the trigger tube will supply to the magnetron pulses per cycle of modulation, Whereas if the modulation frequency is 100 cycles per second, then the number of pulses supplied by the trigger tube to the magnetron would be 500 pulses per `cycle of modulation. It will thus be evident that although the number of pulses per cycle of modulation is varied, there will be a constant number of pulses applied to the magnetron per second, and that this constant number is equal to the frequency of the originating oscillator, namely 50` kilocycles.

The output pulses from the magnetron are of ultra high frequency and may have a duration of less than .2 microsecond pulse assumed above applied to the grid of the trigger Vs. The power output from the magnetron is essentially constant. As mentioned above, the leading edge of the impulses applied to the grid of tube Vc by the output of the limiter serves to trigger oiT the energy in the pulse shaping circuit and this energy is in turn applied to the`magnetrcn load. The energy in the pulse shaping circuit maybe discharged or dissipated in a shorter time than the time interval of the impulse applied to the grid of tube Vs. The magnetron will cease generating oscillations immediately upon the complete discharge of the artificial line. As an example, if the impulses applied to the trigger Ve `50 are each .2 micro'seo-ond, the line may be so designed that it will discharge in about .l microsecond, as a result of which the magnetron will fire or produce oscillations for only .l microsecond.4 It is essential, however, that the impulse applied to the grid of trigger Ve have a time duration sufficiently long to permit the energy in the line pulse shaping circuit to discharge completely, but sufliciently narrow not to prevent the line pulse shaping circuit from starting to recharge. The artificial line must be able to recharge at a time commensurate with the highest modulation frequency involved.

Fig. 4 graphically illustrates, by graphs c and d, the operation of the trigger tube Ve and its associated circuits. Each pulse (graph d) of approximately .l microsecond duration represents the envelope shape for the high frequency carrier in the output of the magnetron. The positions of these pulses with respect to the positive half of the modulation cycle, are shown in graph c.

The receiver for use with the pulse transmission 2 ci' Figs. l and 2 may, if desired, take the form of Fig. 5. This receiver comprises a suitable antenna pick-up 200 which is connected to a frequen'cy'converter 1205. The receivedenergyin the converter 295 beats with the loscillations from a heterodyne oscillator 2li) to provide in the output of the converter tube pulses at the rate of 50 kilocycles per second of a2() megacycle carrier.

These pulses are amplified in intermediate fre- Although I have described a pulsing system wherein; pulses are transmitted during only the positive halvesof the modulation cycle,.it should beunderstood that the principles of the .invention are not limited to such an arrangement because the same principles are applicable to a system wherein symmetrical modulation may be attained by duplication (that is, a composite system) producing ,pulses during both the positive and negative halves of the modulation cycle.

What is claimed is:

vl. In a pulse transmission system, a magnetron, andimeans-for 'supplying said magnetron with pulses of voltage of constant amplitude, said means including an electron discharge device having a cathode, a grid and an anode, a circuit for rbiasing said grid negative relative tosaid cathode to thereby prevent the flow of current through said device,A a pulse shaping circuit in the form of a plurality of articial line sections connected between said anode and a source of unidirectional voltage for charging said line sections, a connection from said cathode to said magnetron, and means for supplying to said grid Variably spaced relatively positive voltage pulses of a magnitude suiiicient for triggering said electron discharge device to pass current at the occurrence of each of said variably spaced positive pulses, said magnetron generating oscillations solely during the time intervals of the pulses of voltage supplied thereto by said device.

2. A pulse transmission system in accordance with claim l, characterized inthisthat each of said variably spaced voltage pulses applied to said grid has a time duration sufficiently long to permit the energy stored in said pulse shaping circuit to discharge substantially completely through 'said device, but suiiiciently short not to prevent said pulse shaping circuit from recharging between said pulses.

3. In a pulse transmission system, a magnetron,

andv means for supplying said magnetron with pulses of voltage of constant amplitude, Isaid means including an electron discharge device having a cathode, a vgrid and an anode, a cirsaid electron discharge device to pass current,l

said pulses having a time duration which is small compared to the interval between them but sufcuit for biasing said device to cut-off, a pulsed() vice, the duration-of said pulses being 4sufliciently-A short, however, not to prevent said pulse shaping.

circuit from recharging to a desired value inthe interval betweenrpulses, lsaid magnetron generating oscillations solely during the time vintervals of the pulses supplied thereto.

4. In a pulse transmission system, a source of sine Vwaves of superaudible frequency, a differentiating amplifier coupled to said source for providing damped pulses of steep Wave front, vrsaid amplifier having a resonant output circuit tuned to a radio frequency, a diiierentiator coupled to the output circuit of said amplier, a saw-tooth generator having a control electrode coupled tor said differentiator, said generator producing. saw-tooth waves under control of the positive peaks of voltage supplied by Isaid differentiator,

a modulator coupled to the output of said sawtooth generator, said modulator being biased to cut-off for the maximum saw-tooth voltage, and a source of sinusoidal voltage waves also coupled to said modulator for enabling said modulator to pass current during the positive portions of said sinusoidal voltage waves, an ultra high frequency oscillator, and means under control of said modulator for keying said oscillatorto Droduce pulses oiultra high frequency energy.

5. In a pulse transmission system,a source of sine waves of ysuperaudible frequency, a diiierentiating amplier coupled to said source for providing damped pulses of steep wave front, rsaid ampliiier having a resonant output circuit tuned to a radi-o frequency, a differentiator coupled to the output of said amplifier, a saw-tooth generator having a control electrodek coupled tosaid diiferentiator, said generator producing sawtooth waves of a lduration less than one microsecond under control of the positive peaks of voltage supplied by said diferentiator, a modulator` coupled to the output of said saw-tooth generator, said modulator being 'biased to cut-off for the maximum saw-tooth voltage, and a source of .sinusoidal voltage waves alsok coupled to said modulator for enabling said modulator to pass current during the positive portions of said sinusoidalv voltage waves, said current being modulated by said saw-tooth waves, means in the output of said modulator for converting the modulations therein to unidirectional pulses of substantially constant amplitude and whose groupings are a function of the instantaneous amplitude of the sinusoidal voltage waves, a keyer connected `to the said means and under control thereof, an energy storage circuit conf which comprises producing equally spaced pulses ciently long to permit the energy 'stored in said n of radio frequency energy recurring at a superaudible rate, converting said pulses'to vunidirectional pulses and producing from said unidirectional pulses other pulses of equal duration which are grouped as a function of the instantaneous amplitude'of a sinusoidal voltage wave.

'7. Ihe methodof operating a pulsing system i which comprises producing equallyl spaced pulses of radio frequency energy recurring at a superaudible rate, converting said pulses to unidirectional pulses each having a duration short compared to the time intervals between them, utilizing said unidirectional pulses to produce other pulses 'which are of equal duration but whose spacing is modulated as a function of the instantaneous amplitude of a signal wave, and radiating equal duration pulses of ultra 'short wave energy of constant power in accordance with the variable spacing between said `modulated pulses.

8. The method of operating a pulsing system which comprises producing equally spaced pulses of radio frequency energy recurring at a superaudible rate, converting said pulses to unidirectional pulses each having a duration sho-rt compared to the time intervals between them, utilizing said unidirectional pulses to produce other pulses which are of equal duration but whose spacing is modulated as a function of the instantaneous amplitude of a signal wave, removing amplitude variations from said space modulated pulses, and producing from said modulated pulses correspondingly positioned pulses of ultra short wave energy of constant power and equal duration.

9. The method of operating a pulsing system which comprises producing equally spaced pulses of radio frequency energy recurring at an appreciably lower rate, converting said pulses to unidirectional pulses each having a duration short compared to the time intervals between them, utilizing said unidirectional pulses to produce other pulses which are of equal duration but whose spacing is modulated as a function of the instantaneous amplitude of a signal Wave, and radiating equal duration pulses of ultra short wave energy of constant power in accordance with the variable spacing between said modulated pulses and at a rate equal to said appreciably lower rate.

10. In a pulse transmission system, a magnetron, and means for supplying said magnetron with pulses of voltage of constant amplitude, said means including an electron discharge device having a cathode, a grid and antanode, a circuit for biasing said grid negative relative to said cathode to thereby prevent the ilow of current through said device, a pulse shaping circuit in the form of a plurality of serially arranged artificial line sections connected between said anode, said line sections comprising series inductance and shunt capacitance, a reactor coil at that end of said pulse shaping circuit farthest removed from said anode, and a source of unidirectional voltage for charging 'said line sections through said reactor coil, a connection from said cathode to the anode of said magnetron, an inductance coil between ground and the junction point of said last connection with said cathode, and means for supplying to said grid variably spaced relatively positive voltage pulses of a magnitude sufficient for triggering said electron discharge device to pass current at the occurrence of each of said. variably spaced positive pulses, said magnetron generating oscillations solely during the time intervals of the pulses of voltage supplied thereto by said device.

11. In combination with an electron discharge device magnetron having a cathode, a surrounding anode constituting a resonant cavity, means for producing a magnetic field parallel to said cathode, a connection from said cathode to ground, means supplying said anode with recurring rectangular wave form pulses ci positive polarity and equal duration and of steep starting and trailing edges, said pulses having a magnitude sufficient to cause said magnetron to produce correspondingly positioned pulsesof high frequency energysolely during the time intervals of said pulses, and means for varying the spacing between the equal duration pulses supplied to said magnetron in accordance with modulating potentials.

JOHN EVANS. 

